text: Many therapeutic proteins in the market are antibodies, which are N-linked glycoproteins. For example, Etanercept, Infliximab and Rituximab are N-glycosylated therapeutic proteins.
  The importance of N-linked glycosylation is becoming increasingly evident in the field of pharmaceuticals. Although bacterial or yeast protein production systems have significant potential advantages such as high yield and low cost, problems arise when the protein of interest is a glycoprotein. Most prokaryotic expression systems such as E. coli cannot carry out post-translational modifications. On the other hand, eukaryotic expression hosts such as yeast and animal cells, have different glycosylation patterns. The proteins produced in these expression hosts are often not identical to human protein and thus, cause immunogenic reactions in patients. For example, S.cerevisiae (yeast) often produce high-mannose glycans which are immunogenic.
  Non-human mammalian expression systems such as CHO or NS0 cells have the machinery required to add complex, human-type glycans. However, glycans produced in these systems can differ from glycans produced in humans, as they can be capped with both N-glycolylneuraminic acid (Neu5Gc) and N-acetylneuraminic acid (Neu5Ac), whereas human cells only produce glycoproteins containing N-acetylneuraminic acid. Furthermore, animal cells can also produce glycoproteins containing the galactose-alpha-1,3-galactose epitope, which can induce serious allergenic reactions, including anaphylactic shock, in people who have Alpha-gal allergy.
  These drawbacks have been addressed by several approaches such as eliminating the pathways that produce these glycan structures through genetic knockouts. Furthermore, other expression systems have been genetically engineered to produce therapeutic glycoproteins with human-like N-linked glycans. These include yeasts such as Pichia pastoris, insect cell lines, green plants, and even bacteria.